Hi,
Please note, that you can use any model inside the 3D Max scene as 
Custom Object under 
Geometry tab. So, if you 
Merge (or link as 
XRef) any model into your scene and then create 
Forest object you can pick it as 
Custom Object. Model linked as 
XRef will be not saved within the scene and everytime needed the model is loaded from it´s external reference file. 
Merge function will import whole geometry so the total size of the scene will increment of model size.
Anyway this process can be done easily in automatic way. For this purpose there is 
XFrog Catalog linking all models through 
Library Browser inside Forest Pack. Please check this 
page about Forest Pack support for XFrog plants.
Anyway, 
XFrog Catalog supports commercial plants packages so in case you have just free samples and you want to use 
Library Browser to insert them, you´ll need to add them to your own personal library. 
Here you can find instructions how to edit your personal libraries. Please also check this reference page describing functions of 
Library Browser.
Also have a good news about Evermotion's Archmodels. We´ll release 
Evermotion Catalog containing most used vegetation packages soon ...
Hope it helps you 
